Coastal Ambient Noise and Microseismic Monitoring with Distributed Acoustic Sensing: a Case Study from Norfolk, UK

Seismic ambient noise is a ubiquitous and constant resource, ideal for non-invasive investigations of the solid earth. Coastlines around the world are handling an increase in coastal erosion due to sea level rise and more energetic storms. Monitoring this is becoming an increasingly necessary task to protect coastal settlements. Using Distributed Acoustic Sensing in seismic monitoring has already shown incredible potential and offers the advantage of dense measurements. Our project seeks to identify the efficacy of Distributed Acoustic Sensing for monitoring subsurface changes which precede cliff failure. We present early findings from the first long-term deployment of a fibre optic cable along the coastline - North Sea, Norfolk, UK. We investigate differences in signal characteristics between conventional seismometers and Distributed Acoustic Sensing in this setting, and interpret the seismic signatures of key sources in the area. This deployment was recording for 22 months, allowing us to monitor both short-term and seasonal changes. We identify the frequency ranges excited by storm events (0.2 - 1 Hz), the dominance of short-period secondary microseismic activity, and the importance of local sea state and weather on influencing higher frequency signals. We also discuss limitations of Distributed Acoustic Sensing and the sources it can not reliably capture when compared to broadband seismometers and nodal geophones. We conclude by discussing how this noise analysis affects the use of ambient noise tomography for seismic velocity monitoring. Future research will test the efficacy of such applications, with the hope of providing better estimates of coastal recession and identifying hazardous areas on a metre-scale.